The invention relates to a blast furnace for iron making, which at least in the hearth portion, comprises a steel plate lining, inside which lining at least one layer of refractory brickwork is arranged, the steel plate lining being joined to the layer (layers) of brickwork by means of mortar joints and/or ramming compound joints to form a cohesive structure. The hearth portion of a blast furnace is often provided with an external cooling system.
In modem large-scale blast faces, in which ever higher iron production levels at elevated gas pressure are reached, it is highly important for the period between two renovations of the brickwork to be as long as possible. This may lead to problems, in particular in the area of the hearth.
Especially in the hearth, the brickwork is exposed both to the action of the gas atmosphere in the furnace and to the action of liquid metal and/or liquid slag materials which are present in that area. The gas atmosphere may lead to a chemical attack on the brickwork, often an alkali attack, while the liquid iron may have a combined influence of high temperature, chemical attack and mechanical attack. This attack is partly caused by the fact that the liquid iron is often not saturated with carbon and therefore tends to dissolve carbon from bricks.
In terms of the structure of the hearth brickwork, it is important that the bricks should not crumble on the hot side at high temperature as a result of their tendency towards thermal expansion. It has been found that carbon-containing materials, such as graphite and semigraphite, are most resistant to crumbling under such circumstances, but the composition of these materials means that they are also susceptible to attack from the liquid iron which may or may not be saturated with carbon. This susceptibility manifests itself primarily by these carbon-containing materials being dissolved in the liquid iron.
It has been found that the bricks are not affected by the liquid iron if a solid layer based on a mixture, in various combinations, of solidified iron, slag and coke particles is able to form on the inside of the brickwork. This so-called xe2x80x9cskullxe2x80x9d forms on the brickwork at a temperature in the region of less than 1100 to 1150xc2x0 C. In addition, the formation of this skull is also dependent on the speed at which the liquid iron is moving into the hearth. Since liquid iron flows out of the heart periodically only at the location of a few tapping points from the furnace, this liquid iron has not only a vertical flow component but also a flow component in the circumferential direction of the furnace, resulting in a higher speed of movement of iron along the brickwork. This iron flowing past has a tendency to redissolve the skull in this area. Only if the hot side of the brickwork can be kept sufficiently cool by means of sufficiently intensive heat dissipation through this brickwork will the skull formed on this brickwork always be sufficient to protect the brickwork from attack.
It should be noted that the xe2x80x9cdead manxe2x80x9d phenomenon often occurs in blast furnaces, i.e. a solid plug based predominantly on coke and iron forms inside the hearth. Especially if this xe2x80x9cdead manxe2x80x9d is extensive and has a low porosity, the circulation speed of liquid iron along the brickwork wall will increase and consequently the attack on the skull will be intensified. This phenomenon also requires an even more intensive dissipation of heat via the brickwork in order to keep the temperature on the hot side of the said brickwork sufficiently low for a skull to remain in place.
Heat dissipation from the hearth brickwork by means of cooling plates which extend deep into the brickwork and through which water flows or by means of so-called xe2x80x9cstave coolersxe2x80x9d arranged inside the steel plate lining is not preferred. Should the skull happen to fall or melt off and part of the brickwork be dissolved in that area, it is possible for liquid iron to come into contact with, for example, such a water-cooled copper cooling plate which extends deep into the brickwork. In such a situation, the copper of the cooling plate may melt through and then the water flowing into the furnace may lead to an explosion followed by rupture of the wall. For these reasons, it is often preferred to provide the steel plate lining of the wall structure with an external cooling feature for the purpose of cooling the hearth. As a rule, this cooling feature is a spray-cooling system with which the temperature of the steel plate lining can be kept at approximately 50xc2x0 C. At a steel plate lining temperature of approximately 50xc2x0 C., it will not always be possible to keep the hot side of the brickwork below a temperature of approx. 1100xc2x0 C., even if bricks made from graphite and/or semigraphite, which have a good thermal conductivity, are used. In this case, it should be noted that the brickwork must have a sufficient thickness to keep the risk of occasional penetration sufficiently low.
It has been found that mortar joints and ramming compound joints form considerable obstacles to the heat dissipation through the brickwork. The outer layer of bricks is generally placed against the steel plate lining with a mortar or ramming compound between them, in which case the thickness of a mortar joint may, for example, be 3 to 5 mm and the thickness of a ramming compound joint may, for example, be 30 to 120 mm. This joint serves partly to compensate for the dimensional deviations of the steel plate lining and partly to bring about thermal contact between steel plate lining and outer brickwork layer. If a plurality of layers of bricks are employed in the radial direction in the wall structure, it will also be necessary to bridge a joint between these layers, and ramming compound is generally employed for this purpose. In any case, like the joint directly behind the steel plate lining, this joint may also serve as an expansion joint. For example, this joint may be 50 mm wide. It has been found that the mortar and/or ramming compound joints may be responsible for 50 to 80% of the total thermal resistance caused by the brickwork to the outer side of the steel plate lining, if the brickwork comprises bricks with a xcex greater than 20w/mxc2x0 C. This problem can become even greater if the structure xe2x80x9cbreathesxe2x80x9d. For example, if there are considerable temperature differences in the steel plate lining, the mortar joint may open up, resulting in an insulating layer of gas. A similar phenomenon may occur if the thermal action of the various bricks causes the joint containing ramming compound to remain insufficiently tight.
The object of the invention is to provide a solution to these problems and, in particular, to improve the heat dissipation from the hot side of the brickwork in such a manner that a skull can continually be formed there. The invention consists in the fact that, in the hearth portion of blast furnace, metal bars which run in the circumferential direction inside the steel plate lining and project into the wall are present, which bars each are connected to the outer side of the steel plate lining by means of two horizontally spaced attachment means each separately running through the steel plate lining, the attachment means being provided with prestressing means for exerting a prestressing force to ensure that each bar always remains pressed against the bricks to maintain a surface-to-surface contact along horizontal and vertical surfaces between the metal bars and bricks during operation. The combination of improved thermal conductivity through the metal bars with a direct surface-to-surface contact between the metal bars and the bricks along horizontal and vertical surfaces, as a result of the attachment with the prestressing means of the metal bars, to a large extent minimizes the thermal resistance of part of the joints. It should be noted that the vertical attachment of the bars is required in order to ensure that, following assembly of the wall structure, the surface-to-surface contact between bars and bricks is maintained if thermal expansion were to allow the bricks to move slightly in the vertical direction.
Also the thermal resistance of the structure is reduced further if the bars are also prestressed in the radial direction with respect to the steel plate lining to maintain a surface-to-surface contact along vertical surfaces with bricks during operation. Any joint which is present can then be reduced to a width of virtually zero, in which case the thermal resistance of this joint is also very low. This latter effect can be obtained in particular if prestressing means are provided in order to hold the bars pressed against the bricks in the radial direction.
Obviously, there is also a thermal resistance between the metal bars and the steel plate lining. However, the effect of this is negligible if, according to the invention, the metal bars are cooled. According to the invention, one possibility for doing this consists in the metal bars and/or their attachment means being designed at least in part as so-called xe2x80x9cheat pipesxe2x80x9d. Heat pipes are generally known construction components in which a liquid and the vapour phase of this liquid are present inside a closed cavity within these construction components. This allows an intensive flow of heat through the heat pipes. According to another embodiment according to the invention, the metal bars are provided with a duct and with feed and discharge means which are connected to a coolant circuit. Direct cooling of the metal bars means that there is no longer any need to dissipate heat from these bars via the steel plate lining. It is preferable for the metal bars to be made from a metal which comprises predominantly copper. This ensures a good thermal conductivity, while the bars provided with a duct can easily be manufactured from copper. It is important that the bars have some individual mobility. Since the thermal movements which have to be absorbed by this mobility are only slight, this does not cause any major design problems. In a possible embodiment according to the invention, the bars inside the steel plate lining are arranged as broken rings and/or in an offset manner. According to another embodiment, the bars inside the steel plate lining form rings which comprise at least 10 and preferably between 30 and 50 bars. According to a possible embodiment of the novel wall structure, the bars have, on the hot wall side, a curved surface which corresponds to the local radius of curvature of the wall. According to another embodiment, the bars may have, on the hot wall side, flat surfaces which together form a regular polygon. This then makes it possible for the bricks also to be provided with flat boundary faces on their outer radial side. As a result, it is possible to obtain a good level of thermal contact between the bars and the bricks which bear against them in the radial direction.
To achieve a good level of surface-to-surface contact along horizontal surfaces between the bars and the bricks and, furthermore, for other design reasons, it is desirable for the bars to extend 15 to 30 cm in the radial direction from the steel plate lining. Furthermore, according to the invention it is preferable for the bars to be positioned vertically at distances of between 40 and 80 cm.
According to a possible embodiment of the novel blast furnace, the brickwork in the radial direction comprises one layer of bricks which are of different lengths and extend to close to the steel plate lining and to against the bars. This design has the advantage that there is no intervening gap containing ramming compound.
According to another advantageous embodiment of the novel blast furnace structure, the brickwork in the radial direction comprises two layers of bricks, between which the joint for each horizontal layer of bricks is offset in the radial direction. In this case, therefore, there is no continuous joint, but rather bricks in the outer layer and in the inner layer bear against one another turn and turn about with surface-to-surface contact along horizontal surfaces. As a result, the thermal conductivity passes directly via these horizontal surfaces from the inner (in the radial direction) layer of bricks to the outer (in the radial direction) layer of bricks.
Where joints are still present in the proposed blast furnace construction, for example between the steel plate lining and the bars, between the steel plate lining and the bricks, and between bricks which adjoin one another in the radial direction, these joints may, according to the invention, be filled with a plastic, highly thermally conductive compound. However, the bricks may also be placed dry against the steel plate lining. A compound of this nature can be obtained if it contains a tar component which evaporates only at high temperature. This tar component ensures that the compound in the joint remains plastic. In the event of the shape of the joint changing, without a concurrent change in volume, the compound, which in itself has good conductivity will maintain good tight contact with the components which form a joint. A further improvement to the thermal conductivity can be obtained if the compound employed also contains a metal or a metal alloy with a melting point or melting range between 200 and 1100xc2x0 C., preferably between 200 and 660xc2x0 C. Tin, for example, melts at approximately 230xc2x0 C., with the result that metallic thermal bridges are then formed in the joint. The same effect can also be obtained by, for example, arranging tin in the joints which run radially between bricks, i.e. in joints between bricks which lie next to one another in the circumferential direction in the same level. Often, bricks will be laid with a thin layer of mortar between them, but the layer of mortar then also forms a thermal bridge. Particularly if the flow of heat does not run in a purely radial direction, such as for example when the furnace is tapped only via a limited number of tapping holes, it is important for there to be no substantial thermal resistance in the circumferential direction of the brickwork.
The novel invention now allows the brickwork to be almost permanently protected by a skull. As a result the risk involved in using graphite and/or semigraphite and/or carbon-containing material with pores of xe2x89xa61 xcexcm and a coefficient of thermal conduction xcex greater than 15w/mxc2x0 C. for the bricks is very considerably reduced, and it is therefore also preferably to employ bricks of this nature, due to the fact that bricks made from these materials only crumble under the influence of thermal stresses at very much higher temperatures than other refractory materials and also have a very high thermal conductivity.
The invention also relates to a method of operating a blast furnace. This method makes it possible, given an identical thickness of the brickwork, to dissipate considerably greater amounts of heat, with the result that it is possible to achieve a lower temperature on the hot side of the brickwork. It is recommended for the flow rate of the liquid circuit through the bars to be set to a heat dissipation of  greater than 50% of the total heat dissipated from the wall.